1. Field of the Invention
This invention relates to a method of grafting molecules onto preformed polymer surfaces.
2. Description of the Prior Art
It is often considered desirable to be able to change the surface characteristics of a polymer surface without affecting the other advantageous physical and chemical characteristics of the polymer. Numerous preformed polymer surfaces, such as those of molded or shaped polymers or of polymer coatings, are dependent upon their surface characteristics for effective utilization in their contemplated applications. Microporous membranes and skinned or molecular filtration membranes are examples of preformed polymers whose effectiveness depends to a great extent upon their surface characteristics.
Microporous membranes are thin sheets having uniform continuous porous structures which function as absolute screens, retaining on their surfaces all bacteria, viruses, macromolecules or particles larger than the pore diameter. Such membranes can be effectively used as filters for cold sterilization in the pharmaceutical and food processing industries.
Skinned membranes consist of thin polymeric films supported on a highly porous structure. Since skinned membranes retain most molecules above a nominal molecular weight limit, these membranes may suitably be used as molecular filters for desalination, concentration, purification of viruses, macromolecules and small molecules, and other similar applications.
To be useful in these applications, microporous and skinned membranes are desirably made from materials which are chemically and biologically inert and exhibit good mechanical properties and structural stability. Fluorocarbon polymers, such as polyvinylidene fluoride, have been found to be particularly suitable.
Hydrophilic membranes which are readily wet by an aqueous solution allow the liquid to flow therethrough, but do not permit the flow of gas, micro-organisms and micron-size particles when wet. Hydrophobic membranes, by contrast, which are not wet by an aqueous solution and do not permit its passage therethrough, allow gas to pass readily while maintaining their effectiveness to filter micro-organisms and micron-size particles. Both hydrophilic and hydrophobic membranes can be used, for example, in the sterilization of liquids being administered intravenously. A hydrophilic membrane placed in the path of the liquid would filter gas and micro-organisms harmful to the patient. A hydrophobic membrane could be employed to vent the filtered gas to the atmosphere without loss of any of the aqueous liquid.
The fluorocarbon polymers such as poly(vinylidene fluoride) which are particularly suitable membrane materials are generally hydrophobic, and therefore, cannot be used in those applications requiring a hydrophilic surface.
Prior to the present invention, a hydrophobic membrane could be treated with a solution of a water-insoluble wetting agent to render it hydrophilic. It has been found, however, that steam sterilization of or passage of water through membranes that had been so treated resulted in the partial loss of their hydrophilic character. Therefore, a method was needed which would render the polymer surface more stably hydrophilic.
It is known that fluorocarbon polymers can be treated with medium-to-strong bases to remove hydrogen fluoride from the polymer chain and crosslink the polymer. See, e.g., Smith, "The Chemistry of Vulcanization of `Viton` A Fluorocarbon Elastomer", Rubber World, Vol. 142, pp. 102-107 (1960); Paciorek et al., "Mechanism of Amine Crosslinking of Fluoroelastomers. I. Solution Studies", J. Polymer Sci., Vol. 65, pp. 405-413 (1960); Mark, Encyclopedia of Polymer Science and Technology, Vol. 14, pp. 612-613 (1971). It is observed by Dwight et al., "Fluoropolymer Surface Studies", J. Colloid Sci., Vol. 47, No. 3, pp. 650-660 (1974) that etching of the surface of a fluorocarbon polymer with a solution of sodium in ammonia removes most of the fluorine and results in a surface which exhibits the characteristics of an unsaturated oxidized hydrocarbon and is relatively unstable.
In U.S. Pat. No. 2,972,586 films of hydrophobic materials such as polyvinyl chloride and copolymers of vinyl chloride and vinylidene chloride are made hydrophilic by the introduction of ionic groups into the film. The reaction involves replacement of halogen atoms in the film forming material by quaternary ammonium or guanidinium groups, of tertiary sulfonium or sulfonic groups.
In U.S. Pat. No. 3,617,344 polymer surfaces are rendered nonthrombogenic by heparin chemically combined with quaternary ammonium groups which are chemically bonded to the polymer surfaces. In one embodiment, a polymer, such as polytetrafluoroethylene, is provided with tertiary amine groups on its surface by radiation grafting of a tertiary amine to the surface, and the chemically bonded tertiary amine groups are then quaternized with a quaternizing agent. The quaternary ammonium groups are then heparinized.
In U.S. Pat. Nos. 3,380,848, 3,401,049, 3,494,862, 3,698,931, 3,880,580, 2,929,800, and 3,940,377 various methods are disclosed in which polymeric surfaces are treated with solutions of silver salts and polymerizable monomers so that silver or silver oxide is deposited on the surface in order to catalyze the grafting and polymerization of the monomer onto the surface.
Although the prior art discloses processes for treating a polymer with either a medium-to-strong base or a substance to be reacted with the polymer, none of these processes discloses a process for subjecting a polymer surface to the action of both the base and the reactant substance for the purpose of grafting the reactant substance onto the surface of the polymer.